STIRLING ENGINE-SCOOTER

Dean Kamen's Segway has been a modest success, but it certainly hasn't become "to the car what the car was to the horse and buggy," as the inventor once predicted. Still, that hasn't stopped Kamen from continuing to invent new types of transportation. The latest is a hybrid electric scooter that uses a Stirling engine to partially recharge its battery pack.

Kamen has already used a Stirling engine--technology that that was invented in 1816, nearly a century before the internal combustion engine--in a car that has been shown off to the public. But while he is rumored to have used the prototype bike on his estate, we still only have drawings to go on.

According to Kamen's patent, the invention appears to be a conventional scooter with a Stirling engine and fuel tank under the seat, a rechargable battery pack in the floor, and a radiator in the front. The Stirling engine has a low energy output on its own, but it could supply power when the bike is at a standstill, making sure that the battery never dies.

The inventor has invested $50 million in Stirling engine technology, which uses two pistons (a power piston and a displacer piston) to turn a crankshaft without using valves. The power piston's cylinder is heated from the outside, forcing gas inside the cylinder to expand, moving the power piston and giving the crankshaft a power stroke. Momentum from the flywheel keeps the engine moving. Instead of shooting expanded gas into the atmosphere, it's sent into the cooled displacer piston, where it contracts and is sent back to the power piston to be heated and used again. In other words, the Stirling engine, which can run on anything that burns, completely uses up whatever fuel is provided. And since the engine doesn't release any unburned hydrocarbons, CO2 emissions are minimal.

Kamen's hybrid bike won't revolutionize anything--plenty of companies are working on hybrid scooters, bicycles, and motorcycles--but it could at the very least help make Stirling engines cool again.